Alternative approach to preventing empty paragraphs from collapsing

Using the ::after pseudo-element with text content provides a more
consistent box height with non-empty paragraphs. This prevents table
cell heights from shifting when the caret iterates through them.
Improve horizontal alignment of caret between characters
Merge pull request #781 from peitschie/empty-p-display

CSS tweaks to improve display of empty paragraphs & tables
Fix broken assert in MergeParagraph op

The operation was attempting to assert that the source step was the
first step within the paragraph as well. Instead, it was only asserting
that the source paragraph was adjacent to the destination paragraph.
Add unit tests showing MergeParagraph OT issue
Fix OT when two adjacent paragraphs are merged backwards

Transforms were not ensuring a consistent style name was applied to the
merged paragraph on all clients.